FIG. 1 illustrates a typical wind turbine 1 for use in large scale electricity generation on a wind farm. The wind turbine 1 includes a tower 2 and a wind turbine nacelle 3 positioned on top of the tower. The wind turbine rotor, comprising three wind turbine blades 5 connected to a hub 4, is connected to the nacelle through a main shaft (not shown in FIG. 1) which extends out of the front of the nacelle. Wind beyond a certain level activates the rotor due to lift induced on the blades and causes the rotor to rotate. The rotation is converted to electric power, which is supplied to the electricity grid.
These tall wind turbines are located on exposed sites to maximize exposure to the wind, but they are also very exposed to lightning strikes which may cause damage to a wind turbine. Thus, wind turbines are typically provided with a lightning protection system.
FIGS. 2a and 2b illustrate a typical lightning protection system in each of the wind turbine blades 5 (a single blade is shown in these Figures). The blades and, in particular, the tips of the blades are the most likely components of a wind turbine to be susceptible to a lightning strike as they are the components that project the highest. Thus, the wind turbine blade 5 of FIG. 2a includes a tip with a metal receptor 8 that intercepts lightning strikes 7 and the receptor is grounded or earthed. The receptor is connected to a lightning down conductor 9 inside the wind turbine blade. The down conductor extends through the blade in the longitudinal direction and ends at the hub 4. FIG. 2b illustrates schematically one known arrangement in which lightning current is transferred from the lightning down conductor 9 to the rest of the lightning protection system and to ground. From the down conductor 9, the lightning current is transferred to the main shaft 10 of the wind turbine through a pitch bearing 13 or any other mechanism between the blade and the shaft. In the nacelle 3, there are sliding contacts or slip rings in contact with the main shaft in order to carry current from the shaft. The slip rings are connected to ground 11 by a down conductor extending through the wind turbine tower 2.
Problems with this type of arrangement are identified in International patent application No. WO2005/050008 in the name of Vestas Wind Systems A/S. These include that the high energy of the lightning current passing through different components of the wind turbine such as the blade pitch bearing 13 and the main shaft bearing may damage these components and that the slip ring arrangement is inefficient.
WO2005/050008 describes an improved lightning current transfer unit to address these problems. It is illustrated in FIGS. 3a to 5.
The lightning current transfer unit 15 of FIG. 3a forms an electrical connection between a lightning down conductor of each blade 5 of the rotor via an electrically conducting ring or blade band 18 around the outside of each blade of the wind turbine and a lightning down conductor of the nacelle 3 via an electrically conducting ring or lightning ring 16 on the nacelle.
As shown best in FIG. 3b, the lightning current transfer unit 15 is mounted on the hub 4 facing the nacelle 3. The lightning current transfer unit 15 projects outwardly from the hub 4 in a space between the wind turbine blade 5 and the front of the nacelle 3. As the lightning current transfer unit 15 is mounted to the hub, it rotates with the hub.
Referring back to FIG. 3a, the electrically conducting or metal band 18 around the outside of each blade 5 surrounds the root of the blade. The band forms a contact surface 18 on the root of the wind turbine blade above the pitching mechanism and perpendicular to the longitudinal direction of the blade. The contact surface thus rotates with pitching of the blade. Each band is connected to the lightning down conductor 9 inside the wind turbine blade as described above.
The conducting ring 16 on the outside of the nacelle 3 facing the hub 4 is mechanically connected to the nacelle. It is electrically connected to a lightning down conductor 14 of the nacelle. The ring 16 forms a nacelle contact surface 17 to the lightning current transfer unit 15.
FIGS. 3b and 3c illustrate in more detail the position of the lightning current transfer unit 15 in relation to the contact surface 18 on the wind turbine blade and the contact surface 17 on the nacelle. It also illustrates the different sections of the lightning current transfer unit 15, which include a base support part 22, two contacts 19a, 19b and flexible links 26, 27 between the base support part and the two contacts. The flexible links ensure that the two contacts are forced against the contact surface 18 on the wind turbine blade 5 and the contact surface 17 of the nacelle 3, respectively. The two contacts 19a, 19b and the two contact surfaces 17, 18 establish two contact areas 20a, 20b. 
The first of the contact areas 20a ensures a contact to the electrically conducting band 18 of the blade 5 and the other 20b to the conducting ring 16 of the nacelle 3. The two contacts 19a, 19b are connected by a dedicated electric connection 30 in the form of a wire or cable.
Each of the contacts 19a, 19b also comprises a retaining bolt 28, 29 for the dedicated electric connection 30 allowing the electric connection to be established and retained between the two contacts. The electric connection 30 is made of a flexible material with a length corresponding to the distance between the two contacts at their rest position, when they are furthest apart. If the lightning connection means or lightning current transfer unit 15 is exposed to forces the flexible link will bend resulting in a more sagging electric connection 30.
FIG. 3d illustrates in cross section the contact 19a. It will be appreciated that that the contact 19b will have the same construction. The contact 19a comprises a metal contact 31 which is in electrical contact with the blade band 18 surrounded by a plastic insulating material 32.
FIG. 4 illustrates schematically the contact areas of the lightning current transfer unit sliding on the contact surface 17 of the conducting ring 16 of the nacelle 3. It illustrates the situation of a rotating three-bladed wind turbine rotor with three lightning current transfer units including contacts 19b. As each lightning current transfer unit is mounted on the hub they will rotate with the main shaft as the centre of rotation. Further, the contacts are positioned at a distance from the centre corresponding to the diameter of the ring 16. The contacts will thus perform a circular rotation facing the ring while being continuously forced against the surface of the ring.
FIG. 5 illustrates the contact surface 18 of the blade 5. The contact 19a is continuously forced against the contact surface or blade band and slides on the surface when the blade is pitched to one or the other side, as indicated by the double headed arrow.
While this arrangement is effective as a lightning current transfer unit, it has been found that under certain conditions the contacts 19a, 19b may bounce off their respective contact surfaces if there are any imperfections in the contact surfaces 17, 18. In addition, the diameter of the blade root on a large turbine may be about 4 meters and this means that the blade band 18 on the blade (if it extends around half the circumference of the blade root) will be over 6 meters long. It is very difficult for this 6 meter long contact surface to have a constant curvature and this means that the contacts may also bounce off the contact surface due to the change in curvature of the contact surface. Under certain site and environmental conditions this may lead to sparks between the contacts and the contact surfaces which in turn may lead to electromagnetic interference. It may be possible to bias the contacts 19a, 19b against the contact surface 18, 17 with a greater force. However, this will lead to high wear on the contacts 19a, 19b with the result that they will have a short lifetime.
In addition the contacts 19a, 19b and the contact surfaces 18, 17 can never be truly smooth as all real surfaces have some form of roughness. The actual area of contact (of a contact 19a, 19b on a respective contact surface 18, 17) is only a small fraction of the total surface area of the contacts 19a, 19b. It can result that the actual contact between a contact 19a, 19b and the respective contact surface 18, 17 is only made by the plastic insulating material 32 and not the metal contact 31. This, under certain conditions, may also lead to sparks between the contact surfaces 18, 17 and the metal contacts 31.
It is an aim of this invention to overcome the problem of the contacts bouncing off their respective contact surfaces. It is also an aim of this invention to ensure that the contacts are in direct electrical connection with their respective contact surfaces.